Voltages from DAC

[Malban]

Hi,

I admit I am not overlay good at electronic stuff - but on the other hand I am interested.

Today I looked at the Vectrex circuits again and came across following section:


(from the Vectrex service manual)
Basicly what you see is the DAC 1408, which translates incoming digital signals (Bit1-8) to analog "current" to Iout.

The analog current gets converted to voltage by 1/2 half of the OpAmp LF 353 (and R 303).
Now - my question, what voltages are generated to IN1 of the MUX (4052b).

Are these voltages from -5V - +5V or 0V - 10V (or something completely different).

(Also - what exactly does Capacitor C 315 do? I wouldn't expect a capacitor in a current to voltage circuit).

Thx

Malban

[Kristof]

Hi Malban,

The voltages of the Opamp output are approximately +2,35Volt (PortA = 0x7F) to -2,35Volt (PortA = -0x7F). I measured it on one of my Vectrex-units using a scope.

My opinion of C315 is that it serves as a kind of lowpass filter in order to filter away the glitches in the output when the 8 DAC input-lines are changing their values.

Regards,

Kristof

[Malban]

Thanks for the info.

I have no osciloscope - or multimeter for that matter (and if I did I'd probably have fried different devices by now). Never would have guessed the range you measured...

Malban

[gauze]

I was reading about opamps and this explanation of the cap might apply here (analog electronics give me a headache):

"The capacitor (if chosen to be large enough) reduces to a neglible value the phase shift in the feedback due to input capacitance of the op-amp (with R(esistor)) that could reduce the phase margin and potentially cause instability. So it eliminates a problem that is caused by using R(esistor) in the feedback rather than a short."

[kokovec]

Yep, it looks like the OpAmp is configured as a low-pass filter.